inheritable_class_variables.rb

  def self.attr_inheritable(*variables)
    variables.each do |v|
      module_eval %{
        def self.#{v}
          @#{v} = superclass.#{v} if !instance_variable_defined?(:@#{v}) && superclass.respond_to?(:#{v})
          return @#{v}
        end
      }
    end
  end